netdev - Re: [PATCH bpf-next V2] xdp: bpf_xdp_metadata use NODEV for no device support

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Date:   Fri, 17 Feb 2023 10:01:40 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Martin KaFai Lau <martin.lau@...ux.dev>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, martin.lau@...nel.org, ast@...nel.org,
        daniel@...earbox.net, alexandr.lobakin@...el.com,
        larysa.zaremba@...el.com, xdp-hints@...-project.net
Subject: Re: [PATCH bpf-next V2] xdp: bpf_xdp_metadata use NODEV for no device support

On Fri, Feb 17, 2023 at 9:55 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 2/17/23 9:40 AM, Stanislav Fomichev wrote:
> > On Fri, Feb 17, 2023 at 9:39 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> >>
> >> On 2/17/23 9:32 AM, Stanislav Fomichev wrote:
> >>> On 02/17, Jesper Dangaard Brouer wrote:
> >>>> With our XDP-hints kfunc approach, where individual drivers overload the
> >>>> default implementation, it can be hard for API users to determine
> >>>> whether or not the current device driver have this kfunc available.
> >>>
> >>>> Change the default implementations to use an errno (ENODEV), that
> >>>> drivers shouldn't return, to make it possible for BPF runtime to
> >>>> determine if bpf kfunc for xdp metadata isn't implemented by driver.
> >>>
> >>>> This is intended to ease supporting and troubleshooting setups. E.g.
> >>>> when users on mailing list report -19 (ENODEV) as an error, then we can
> >>>> immediately tell them their device driver is too old.
> >>>
> >>> I agree with the v1 comments that I'm not sure how it helps.
> >>> Why can't we update the doc in the same fashion and say that
> >>> the drivers shouldn't return EOPNOTSUPP?
> >>>
> >>> I'm fine with the change if you think it makes your/users life
> >>> easier. Although I don't really understand how. We can, as Toke
> >>> mentioned, ask the users to provide jited program dump if it's
> >>> mostly about user reports.
> >>
> >> and there is xdp-features also.
> >
> > Yeah, I was going to suggest it, but then I wasn't sure how to
> > reconcile our 'kfunc is not a uapi' with xdp-features (that probably
> > is a uapi)?
>
> uapi concern is a bit in xdp-features may go away because the kfunc may go away ?

Yeah, if it's another kind of bitmask we'd have to retain those bits
(in case of a particular kfunc ever going away)..

> May be a list of xdp kfunc names that it supports? A list of kfunc btf id will
> do also and the user space will need to map it back. Not sure if it is easily
> doable in xdp-features.

Good point. A string list / btf_id list of kfuncs implemented by
netdev might be a good alternative.